At the Edge of Stability: the Unexpected Isolation of an f-Element Triphenylphosphine Complex

While PPh3 is a dominant ligand in transition-metal chemistry, it is completely absent in rare-earth and actinide structural chemistry. The open coordination site and high Lewis acidity of (CpSiMe3)3U are leveraged to generate the first structurally characterized PPh3 complex in these series. X-ray crystallography and variable-temperature nuclear magnetic resonance spectroscopy give insight into the structure and fluxional behavior of (CpSiMe3)3U-PPh3 (1). The U-P bonding energetics were experimentally determined to be endergonic at room temperature, partially rationalizing the absence of triphenylphosphine in f-element chemistry. The structural and dynamic properties of 1 were compared to an analogous phosphite complex, (CpSiMe3)3U-P(OCH2)3CEt (2), and the large discrepancies discussed in the context of π-bonding and steric interactions.